1. Field of the Invention
The present invention relates to a power supply apparatus. More specifically, the present invention relates to an improvement in a power supply apparatus for use in a fluorescent lamp starting apparatus.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing one example of a conventional power supply apparatus and FIG. 2 is a graph showing waveforms of the electrical signals at various portions in the power supply apparatus shown in FIG. 1.
Referring to FIG. 1, an alternating voltage source 1 is connected to a rectifying circuit 2 including a diode bridge, in which an alternating voltage V.sub.E shown as the waveform (a) in FIG. 2 is rectified to provide a ripple voltage shown as the waveform (c) shown by the dotted line in FIG. 2. A smoothing capacitor 3 is connected to the output of the rectifying circuit 2. The ripple voltage of the output from the rectifying circuit 2 is smoothed by the smoothing capacitor 3 and a direct current voltage V.sub.O having the ripple component removed and having an approximately constant level as shown as the waveform (c) in FIG. 2 is obtained. The direct current voltage V.sub.O thus obtained is supplied to a load 4. Meanwhile, an input current I.sub.S shown as the waveform (b) in FIG. 2 flows from the alternating current voltage source 1 to the rectifying circuit 2.
Referring to the power supply apparatus shown in FIG. 1, assuming that there is no distortion in the alternating current source voltage, the input power factor may be expressed by the following equation: ##EQU1## where I.sub.1 is a fundamental component when the input current I.sub.S is expanded in a Fourier series, I.sub.n is an n-th harmonic component when the input current I.sub.S is expanded in a Fourier series, and .phi. is a phase difference between the fundamental component I.sub.1 when the input current I.sub.S is expanded in a Fourier series and the fundamental component of the source voltage V.sub.E.
More specifically, as seen from the above described equation, the input power factor of the power supply apparatus shown in FIG. 1 is much less influenced by the phase difference of the current and voltage of the fundamental component and is mainly determined by the factor determinable by the distortion factor of ##EQU2## Therefore, a power supply apparatus of a capacitor input type in which an input current I.sub.S of a pulsive form flows involved a disadvantage that the input power factor is poor and the effective value of the input current I.sub.S becomes large. The power supply apparatus shown in FIG. 1 also involved a disadvantage that when the alternating current voltage V.sub.E from the alternating current voltage source 1 is supplied to the rectifying circuit 2 a surge current flowing into the smoothing capacitor 3 becomes large.
Recently various types of power supply apparatuses eliminating the above described disadvantages were proposed. One example is disclosed in, for example, U.S. Pat. No. 3,987,356 entitled "Controlled Capacitive Filter for Active Loads" and issued Oct. 19, 1976 to Robert Steigerwald.
FIG. 3 is a schematic diagram of a power supply apparatus in accordance with the above described U.S. Pat. No. 3,987,356 and FIG. 4 is a graph showing waveforms of electrical signals for explaining the operation of the power supply apparatus shown in FIG. 3.
Now the circuit configuration of the power supply apparatus shown in FIG. 3 will be described. An alternating current voltage source 1 is connected to the input of a rectifying circuit 2. A series connection of a diode 5, a smoothing capacitor 3, an inductance 6, and a first control switch 7 of such as a gate controlled thyristor or a gate turn-off thyristor is connected to the output of the rectifying circuit 2. The inductance 6 serves to limit an increase rate of a current flowing through the control switch 7 on the occasion of turn-on of the control switch 7 to a safe value. A load 4 is connected in parallel with the series connection of the smoothing capacitor 3, the inductance 6 and the first control switch 7. A second control switch 10 is connected in parallel with the series connection of the diode 5, the smoothing capacitor 3 and the inductance 6. The second control switch 10 may be a gate controlled thyristor, for example. A series connection of resistors 11 and 12 for dividing the output voltage V.sub.DC is connected to the output terminal of the rectifying circuit 2, so that the divided voltage V.sub.D divided by these resistors 11 and 12 may be applied to a comparison input terminal of a comparator 13. A reference voltage V.sub.CO is supplied to a reference input terminal of the comparator 13. A primary winding of a transformer 14 is connected to the output of the comparator 13 and a secondary winding of the transformer 14 is connected to the cathode and gate of the second control switch 10. A current transformer 8 is provided for detecting a current flowing through the diode 5 and the current detected by the current transformer 8 is supplied through the delay circuit 9 to the gate of the first control switch 7. The delay circuit 9 is provided for delaying the conduction timing of the control switch 7 so that the control switches 7 and 10 may not be rendered conductive simultaneously.
Now referring to FIG. 4, the operation of the power supply apparatus shown in FIG. 3 will be described. The alternating current voltage V.sub.E shown as the waveform (a) in FIG. 4 is rectified by the rectifying circuit 2 to provide a ripple voltage V.sub.DC shown as the waveform (b) by the dotted line in FIG. 4. The first control switch 7 is rendered conductive during the peak of the ripple voltage V.sub.DC, so that a direct current flows from the rectifying circuit 2 through the diode 5, the smoothing capacitor 3, the inductance 6, and the control switch 7, whereby the smoothing capacitor 3 is charged. The voltage V.sub.DC rectified during that period is supplied through the diode 5 to the load 4. When the smoothing capacitor 3 is charged to the peak value of the ripple voltage V.sub.DC, the current flowing into the smoothing capacitor 3 is reversed to supply energy to the load 4, whereby the control switch 7 is naturally turned off. Thereafter the ripple voltage V.sub.DC decreases to a given level at the timing t1 shown in the waveform (b) in FIG. 4. More specifically, if and when the divided voltage V.sub.DC divided by the resistors 11 and 12 becomes smaller than the reference voltage V.sub.C, the comparator 13 provides an output voltage to the primary winding of the transformer 14. A voltage is induced in the secondary winding of the transformer 14 and the induced voltage is supplied to the gate of the control switch 10, whereby the second control switch 10 is turned on. Since the diode 5 has been supplied with a reverse voltage as a function of the terminal voltage of the smoothing capacitor 3 at that time, it follows that the smoothing capacitor 3 is substantially inserted in series with the load 4. Accordingly, at the timing t1 the voltage V.sub.O supplied to the load 4 rapidly increases to a level equal to a sum of the peak value of the ripple voltage, i.e. a terminal voltage of the smoothing capacitor 3 and the ripple voltage V.sub.DC at the timing t1. Since the inductance 6 serves to limit the increase rate of the current flowing through the control switch 10, a surge of a large current flowing into the smoothing capacitor 3 and the load 4 is prevented.
After an initial rapid increase of the voltage in the smoothing capacitor 3 at the timing t1, the smoothing capacitor 3 is discharged, whereby energy is supplied to the load 4, and is completely discharged at the timing t2. When the terminal voltage of the smoothing capacitor 3 becomes 0 V at the timing t2, the diode 5 is again forward biased to be rendered conductive, with the result that a current starts flowing into the load 4. When a load current flows through the diode 5, the flow of the current is detected by the current transformer 8 and the detected signal is supplied to the gate of the control switch 7 with a slight delay as compared with the timing t2 by means of the delay circuit 9 implemented by a one-shot multivibrator, for example, whereby the control switch 7 is turned on.
Thereafter likewise the voltage V.sub.DC obtained through rectification between the timings t2 and t3 is supplied to the load 2 and the smoothing capacitor 3 is charged during a period from the timing slightly delayed from the timing t2 to the timing of the peak value of the ripple voltage V.sub.DC. When the control switch 10 is rendered conductive at the timing t3 the energy stored in the smoothing capacitor 3 is supplied to the load 4.
As described in the foregoing, in the power supply apparatus shown in FIG. 3, the voltage V.sub.O supplied to the load 4 has a low frequency ripple voltage decreased and a conduction period of the input current prolonged as shown as a waveform (b) in FIG. 4, whereby the input power factor is also improved. However, since the power supply voltage shown in FIG. 3 requires the control switches 7 and 10, the inductance 6 for preventing a surge current, the comparator 13, the delay circuit 9 and the like, a disadvantage was involved that the circuit configuration becomes considerably complicated.